The Mysterium cards actually have a careful design behind them - each card has something like 7 elements deliberately included, to make sure that there are plenty of dream cards that match any given person/place/thing card. Using Dixit cards instead of the dream cards means you miss out on those deliberate links. On the other hand, with imagination and good rapport, you can still make, and convey, connections between the Dixit cards and the person/place/thing cards.